Hydrogenation of polynuclear aromatic compounds

ABSTRACT

A process for reducing the polynuclear aromatics (PNA) content of a sulfur-containing hydrocarbon stream. The process includes contacting the sulfur-containing hydrocarbon stream with a dearomatization composition comprising a promoter metal component and zinc oxide. The dearomatization composition has enhanced resistance to sulfur poisoning and is also effective to remove sulfur from the hydrocarbon stream.

BACKGROUND

This invention relates generally to the hydrogenation of polynuclear aromatic compounds (PNA) in low-sulfur hydrocarbon-containing streams, such as diesel fuel. In another aspect, this invention relates to compositions suitable for use in the desulfurization and dearomatization of diesel fuels while increasing the cetane number of the fuel.

Diesel fuel inherently contains certain amounts of aromatic hydrocarbon compounds (i.e., hydrocarbon compounds containing one or more “benzene-like” ring structures). Commercial diesel fuel is typically made by blending “straight run” diesel (produced by simple distillation of crude oil) and a catalytically cracked stock (typically cracked residual oil). Much of the aromatic content of blended diesel fuels originates from the cracked stock because catalytic cracking increases aromatics content.

Aromatic hydrocarbons have poor self-ignition qualities, so that diesel fuels containing a high fraction of aromatics tend to have low cetane numbers. Typical cetane values for straight run diesel are in the range of 50 to 55, while the cetane value for highly aromatic blended diesel fuel is typically in the range of 40 to 45. These low cetane values can produce more difficulty in cold engine starting and can increase combustion noise. The ignition delay experienced with low cetane diesel fuel can also cause increased hydrocarbon emissions and increased NOx emissions.

Polynuclear aromatics (PNA), also called polycyclic aromatic hydrocarbons (PAH), are a type of aromatic compound whose presence in diesel fuel is particularly undesirable. PNA are aromatic compounds having more than one aromatic ring. Some tests have indicated that certain PNA compounds can have adverse health affects on animals. Thus, it is desirable to limit PNA emissions from vehicles, especially in densely populated high traffic urban areas. Scientists have found that a linear relationship exists between fuel PNA input and PNA emissions. Thus, by reducing fuel PNA content in commercially available diesel fuel, PNA emissions to the environment will be reduced.

A variety of commercial processes exist for reducing the total aromatic and PNA content of diesel fuel. The most popular dearomatization processes involve hydrogenation of aromatics in the presence of a catalyst from Group VIII of the Periodic Table of Elements. However, conventional Group VIII catalysts are very susceptible to sulfur poisoning from sulfur-containing compounds in diesel fuel, even when the diesel subjected to hydrodearomatization is low sulfur diesel containing less than 30 parts per million by weight (ppmw) sulfur.

OBJECTS AND SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an improved process for reducing the polynuclear aromatics (PNA) content of hydrocarbon-containing streams that employs a catalyst having a high sulfur tolerance.

Accordingly, one aspect of the present invention concerns a process for removing polynuclear aromatics from a low-sulfur hydrocarbon-containing stream containing less than about 500 parts per million by weight (ppmw) sulfur. This process comprises contacting the low-sulfur stream with a dearomatization composition comprising a promoter metal component and zinc oxide under dearomatization conditions sufficient to reduce the PNA content of the low-sulfur stream by at least about 25 percent by weight thereby providing a PNA-reduced stream, wherein the sulfur content of the PNA-reduced stream is at least about 5 percent by weight less than the sulfur content of the low-sulfur stream.

Another aspect of the present invention concerns a process for treating an initial hydrocarbon-containing stream having a sulfur content of at least about 500 ppmw and a PNA content of at least about 5 percent by weight, the process comprising: (a) contacting the initial stream with a catalyst and/or sorbent composition in a first zone under desulfurization conditions sufficient to reduce the sulfur content of the initial stream by at least about 25 percent by weight, thereby producing a sulfur-reduced stream having a sulfur content of less than about 500 ppmw and a PNA content of at least about 5 percent by weight; and (b) contacting at least a portion of the sulfur-reduced stream with a dearomatization composition comprising nickel and zinc oxide in a second zone to thereby produce a PNA-reduced stream having a PNA content at least about 25 percent by weight less than the PNA content of the sulfur-reduced stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 plots the PNA hydrogenation activity of three different compositions as a function of the amount of sulfur across the catalyst bed.

FIG. 2 compares the resistance to deactivation of two different compositions by plotting the amount of sulfur contained in a feed stream as a function of the amount of sulfur across the catalyst bed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention concerns a dearomatization composition with enhanced tolerance to sulfur poisoning. The dearomatization composition generally comprises a promoter metal component and zinc oxide.

The promoter metal component employed in the dearomatization composition comprises, consists of, or consists essentially of a promoter metal. The promoter metal is a metal capable of catalyzing a hydrogenation reaction, such as hydrogenation of polynuclear aromatics (PNA). Preferably, the promoter metal is selected from the group consisting of nickel, cobalt, iron, manganese, copper, zinc, molybdenum, tungsten, silver, tin, antimony, vanadium, gold, platinum, ruthenium, iridium, chromium, palladium, titanium, zirconium, rhodium, rhenium, and combinations thereof. Most preferably, the promoter metal is nickel.

In one embodiment of the present invention, substantially all of the promoter metal component is present in the dearomatization composition in a reduced-valence state. As used herein, “reduced-valence state” denotes a state of a composition/component where the number of oxygen atoms associated therewith has been reduced. Preferably, substantially all of the promoter metal component is present in a zero valence state, having no oxygen atoms associated therewith. Accordingly, as will be discussed in further detail below, it is preferred for the dearomatization composition to be subjected a reduction step during its preparation. In addition, it is preferred for the dearomatization composition to be subjected to an acid treatment step during its preparation, preferably pretreatment with tartaric acid and/or citric acid, most preferably pretreatment with citric acid.

In one embodiment of the present invention, the promoter metal component of the dearomatization composition comprises, consists of, or consists essentially of, a substitutional solid metal solution containing the promoter metal in solid solution with another metal. The substitutional solid metal solution preferably is characterized by the formula: M_(A)Zn_(B), wherein M is the promoter metal, Zn is zinc, and A and B are each numerical values in the range of from 0.01 to 0.99. In the above formula for the substitutional solid metal solution, it is preferred for A to be in the range of from about 0.70 to about 0.97, and most preferably in the range of from about 0.85 to about 0.95. It is further preferred for B to be in the range of from about 0.03 to about 0.30, and most preferably in the range of from about 0.05 to 0.15. Preferably, B is equal to (1−A).

Substitutional solid solutions have unique physical and chemical properties that are important to the chemistry of the dearomatization composition described herein. Substitutional solid solutions are a subset of alloys that are formed by the direct substitution of the solute metal for the solvent metal atoms in the crystal structure. For example, it is believed that the substitutional solid metal solution (M_(A)Zn_(B)) found in the dearomatization composition is formed by the solute zinc metal atoms substituting for the solvent promoter metal atoms. There are three basic criteria that favor the formation of substitutional solid solutions: (1) the atomic radii of the two elements are within 15 percent of each other; (2) the crystal structures of the two pure phases are the same; and (3) the electronegativities of the two components are similar. The promoter metal (as the elemental metal or metal oxide) and zinc oxide employed in the dearomatization composition described herein preferably meets at least two of the three criteria set forth above. For example, when the promoter metal is nickel, the first and third criteria, are met, but the second is not. The nickel and zinc metal atomic radii are within 10 percent of each other and the electronegativities are similar. However, nickel oxide (NiO) preferentially forms a cubic crystal structure, while zinc oxide (ZnO) prefers a hexagonal crystal structure. A nickel-zinc solid solution retains the cubic structure of the nickel oxide. Forcing the zinc oxide to reside in the cubic structure increases the energy of the phase, which limits the amount of zinc that can be dissolved in the nickel oxide structure. This stoichiometry control manifests itself microscopically in a 92:8 nickel zinc solid solution (Ni_(0.92)Zn_(0.08)) that is formed when the nickel oxide-zinc oxide solid solution is reduced.

In addition to zinc oxide and the reduced-valence promoter metal component, the dearomatization composition may further comprise a porosity enhancer and a promoter metal-zinc aluminate substitutional solid solution. The promoter metal-zinc aluminate substitutional solid solution can be characterized by the formula: M_(Z)Zn_((1-Z))Al₂O₄₎, wherein Z is a numerical value in the range of from 0.01 to 0.99.

The porosity enhancer, when employed, can be any compound which ultimately increases the macroporosity of the dearomatization composition. Preferably, the porosity enhancer is perlite. The term “perlite” as used herein is the petrographic term for a siliceous volcanic rock which naturally occurs in certain regions throughout the world. The distinguishing feature, which sets it apart from other volcanic minerals, is its ability to expand 4 to 20 times its original volume when heated to certain temperatures. When heated above 1,600° F., crushed perlite expands due to the presence of combined water with the crude perlite rock. The combined water vaporizes during the heating process and creates countless tiny bubbles in the heat softened glassy particles. It is these diminutive glass sealed bubbles which account for its light weight. Expanded perlite can be manufactured to weigh as little as 2.5 lbs per cubic foot. Typical chemical analysis properties of expanded perlite are: silicon dioxide 73%, aluminum oxide 17%, potassium oxide 5%, sodium oxide 3%, calcium oxide 1%, plus trace elements. Typical physical properties of expanded perlite are: softening point 1,600-2,000° F., fusion point 2,300° F.-2,450° F., pH 6.6-6.8, and specific gravity 2.2-2.4. The term “expanded perlite” as used herein refers to the spherical form of perlite which has been expanded by heating the perlite siliceous volcanic rock to a temperature above 1,600° F. The term “particulate expanded perlite” or “milled perlite” as used herein denotes that form of expanded perlite which has been subjected to crushing so as to form a particulate mass wherein the particle size of such mass is comprised of at least 97% of particles having a size of less than 2 microns. The term “milled expanded perlite” is intended to mean the product resulting from subjecting expanded perlite particles to milling or crushing.

The dearomatization composition preferably comprises zinc oxide, the reduced-valence promoter metal component (M_(A)Zn_(B)), the porosity enhancer (PE), and the promoter metal-zinc aluminate (M_(Z)Zn_((1-Z))Al₂O₄) in the ranges provided below in Table 1.

TABLE 1 Components of the Dearomatization Composition M_(A)Zn_(B) PE M_(Z)Zn_((1−Z))Al₂O₄ Range ZnO (wt %) (wt %) (wt %) (wt %) Preferred  5–80  5–80 2–50 1–50 More Preferred 20–60 20–60 5–30 5–30 Most Preferred 30–50 30–40 10–20  10–20 

The dearomatization composition is preferably in the form of solid particles suitable for used in a fixed bed or moving bed reactor. The dearomatization composition can be in one or more of the following forms: a granule, an extrudate, a tablet, a sphere, a pellet, or a microsphere. Preferably, the dearomatization composition is in the form of granules, extrudates, tablets, spheres, or pellets having an average minimum particle diameter of at least about 0.0625 inches, more preferably at least 0.125 inches.

The dearomatization composition described above is preferably made by a process comprising the following steps:

-   -   (a) admixing a liquid, a zinc-containing compound, a porosity         enhancer, and an aluminum-containing compound so as to form a         wet mixture;     -   (b) shaping the wet mixture so as to form shaped base particles;     -   (c) drying the shaped base particles so as to form dried base         particles;     -   (d) calcining the dried base particles so as to form calcined         base particles;     -   (e) incorporating a promoter metal into and/or onto the calcined         base particles so as to form promoted particles;     -   (f) contacting the promoted particles with an acid so as to form         acid-treated particles;     -   (g) drying the acid-treated particles so as to form dried,         acid-treated particles;     -   (h) calcining the dried, acid-treated particles so as to form         calcined, acid-treated particles;     -   (i) reducing the calcined, acid-treated particles so as to form         a reduced-valence dearomatization composition;     -   (j) contacting the reduced-valence dearomatization composition         with an acid, so as to form an acid-treated, reduced-valence         dearomatization composition; and     -   (k) drying the acid-treated, reduced-valence dearomatization         composition so as to form a final dearomatization composition         suitable for use in a process for reducing the PNA content of a         hydrocarbon-containing stream, such as diesel fuel.

Steps (a)-(k) are described in further detail below.

To perform step (a), a liquid, a zinc-containing compound, a porosity enhancer, and an aluminum-containing compound are admixed in appropriate proportions by any suitable method or manner which provides for the intimate mixing of such components to thereby provide a substantially homogenous mixture thereof. The term “admixing,” as used herein, denotes mixing components in any order and/or any combination or subcombination. Any suitable means for admixing the components of the composition can be used to achieve the desired dispersion of such components. Examples of suitable admixing means include, but are not limited to, mixing tumblers, stationary shelves or troughs, Eurostar mixers, which are of the batch or continuous type, impact mixers, and the like.

The liquid employed in step (a) can be any liquid capable of dispersing the zinc-containing compound, the porosity enhancer, and the aluminum-containing compound. Preferably, the liquid is selected from the group consisting of water, ethanol, acetone, and combinations thereof. Most preferably, the liquid is water. The zinc-containing compound employed in step (a) can be in the form of zinc oxide or in the form of one or more zinc compounds that are convertible to zinc oxide under the conditions of preparation described herein. Examples of suitable zinc-containing compounds include, but are not limited to, zinc sulfide, zinc sulfate, zinc hydroxide, zinc carbonate, zinc acetate, zinc nitrate, and the like, and combinations thereof. Preferably, the zinc-containing compound is powdered zinc oxide.

The porosity enhancer employed in step (a) can be any compound suitable for enhancing the porosity and/or reducing the density of the final dearomatization compound. Preferably, the porosity enhancer is a silica-containing material such as, for example, expanded crushed perlite, diatomite, silica colloid, silica gel, and/or precipitated silica. In addition, silicon compounds convertible to silica such as, for example, silicic acid, ammonium silicate, and the like can also be employed. Most preferably, the porosity enhancer is crushed expanded perlite.

The aluminum-containing compound employed in step (a) can be any compound that contains aluminum such as, for example, an alumina or an aluminate. Preferably, the aluminum-containing compound is an alumina that can be at least partially converted to an aluminate (e.g. zinc aluminate) upon calcination. Examples of suitable aluminum-containing compounds include aluminum chlorides, aluminum nitrates, colloidal alumina solutions, hydrated aluminas, peptized aluminas, and, generally, those alumnia compounds produced by the dehydration of alumina hydrates. Most preferably, the aluminum-containing compound is a hydrated alumina such as, for example, bohemite or pseudobohemite.

The dry components (i.e., zinc-containing compound, porosity enhancer, and aluminum-containing compound) are preferably employed in the amounts listed below in Table 2. The listed amounts are percentages of the total dry components, excluding the liquid.

TABLE 2 Dry Components of the Base Mixture Zn Compound PE Al Compound Range (wt %) (wt %) (wt %) Preferred 10–95  5–50 1–40 More Preferred 30–85 10–45 5–35 Most Preferred 40–80 12–40 8–25

The amount of liquid added to the dry components is any amount necessary to form a wet mixture of desired consistency. The resulting wet mixture is preferably in the form of a wet mix, dough, paste, or slurry.

In accordance with step (b), the wet mixture resulting from step (a) is shaped to form base particles selected from the group consisting of granules, extrudates, tablets, spheres, pellets, or microspheres. Preferably, the resulting wet mixture is in the form of a dough or a paste, and can be shaped by extrusion to form particles having a minimum particle diameter of at least about 0.0625 inches, and more preferably having a minimum particle diameter in the range of from about 0.125 to about 0.5 inches.

In accordance with step (c), the resulting base particles are then dried under drying conditions, described immediately below, to form dried base particles. The drying conditions, as referred to herein, can include a temperature in the range of from about 150 to about 450° F., preferably in the range of from about 190 to about 410° F., and most preferably in the range of from 200 to 350° F. Such drying conditions can also include a time period generally in the range of from about 0.5 to about 60 hours, preferably in the range of from about 1 to about 40 hours, and most preferably in the range of from 1.5 hours to 20 hours. Such drying conditions can also include a pressure generally in the range of from about atmospheric (i.e., about 14.7 pounds per square inch absolute) to about 150 pounds per square inch absolute (psia), preferably in the range of from about atmospheric to about 100 psia and, most preferably about atmospheric, so long as the desired temperature can be maintained. Any drying method(s) known to one skilled in the art such as, for example, air drying, and/or heat drying can be used. Preferably, heat drying is used.

In accordance with step (d), the dried base particles are then calcined to form calcined base particles. Preferably, calcination is carried out in an oxidizing atmosphere, such as in the presence of oxygen or air. The calcining conditions, as referred to herein, can include a temperature in the range of from about 400 to about 1,500° F., preferably in the range of from about 750 to about 1,450° F., and most preferably in the range of from 750 to 1,400° F. Such calcining conditions can also include a pressure, generally in the range of from about 7 to about 750 psia, preferably in the range of from about 7 psia to about 450 psia, and most preferably in the range of from 7 psia to 150 psia, and a time period in the range of from about 1 hour to about 60 hours, preferably for a time period in the range of from about 1 hour to about 20 hours, and most preferably for a time period in the range of from 1 to 15 hours. In the process of this embodiment, the calcination preferably converts at least a portion of the aluminum-containing compound (e.g., alumina) to an aluminate (e.g., zinc aluminate).

In accordance with step (e), a promoter metal is then incorporated onto and/or into the calcined base particles so as to form promoted particles. A preferred method of incorporating the promoter metal is to impregnate the calcined base particles using incipient wetness impregnation (i.e., substantially filling the pores of the calcined base particles with a solution containing the promoter metal). A preferred impregnating solution used to incorporate the promoter metal component onto and/or into the calcined base particles comprises an aqueous solution of a compound containing the promoter metal. Preferably such promoter metal-containing compound is a metal salt such as a metal chloride, a metal nitrate, or a metal sulfate. The concentration of the promoter metal in the solution can be in any suitable range necessary to incorporate the desired amount of the promoter metal. It is preferred for the calcined base particles to be impregnated with a nickel component by use of a solution containing nickel nitrate hexahydrate dissolved in water so that the final dearomatization composition contains at least 10 weight percent nickel.

In accordance with step (f), the promoted particles are then contacted with an acid to thereby produce acid-treated particles. The acid with which the promoted particles are contacted is preferably selected from the group consisting of citric acid, tartaric acid, and combinations thereof. Preferably, acid treatment is carried out by the impregnation method, as described above. Like the promoter, the acid is dissolved in a solvent, preferably water, to form the impregnating solution.

In accordance with step (g), the acid-treated particles are then subjected to drying under drying conditions, as described above, to thereby form dried, acid-treated particles.

In accordance with step (h), the dried, acid-treated particles are then calcined under calcining conditions, as described above, to thereby form calcined, acid-treated particles. Preferably, calcination causes at least a portion of the zinc oxide and at least a portion of the promoter metal to form a substitutional solid solution.

In accordance with step (i), the calcined, acid-treated particles are then subjected to reduction to thereby form a reduced-valence dearomatization composition. Suitable reducing agents include any reducing agent that causes a reduction in the number of oxygen atoms associated with the calcined, acid-treated particles. Preferably, hydrogen is employed as the reducing agent. The reducing conditions can include a temperature in the range of from about 100 to about 1,500° F., a pressure in the range of from about 15 to about 1,500 psia, and a time sufficient to permit the formation of a reduced-valence promoter metal component.

In accordance with step (j), the reduced-valence dearomatization composition is then treated with an acid to provide an acid-treated, reduced-valence dearomatization composition. The acid with which the reduced-valence dearomatization composition is contacted is preferably selected from the group consisting of citric acid, tartaric acid, and combinations thereof. Most preferably, the acid is citric acid. Preferably, acid treatment is carried out by the impregnation method, as described above.

In accordance with step (k), the acid-treated, reduced-valence dearomatization composition is preferably dried under the drying conditions, described above, to thereby provide the final dearomatization composition.

In an alternative embodiment of the present invention, steps (e)-(h) are repeated one or more times prior to performing steps (i)-(k). In certain instances, the multiple promoter metal incorporation steps carried out when repeating steps (e)-(h) may be necessary to incorporate high levels of the promoter metal. In another alternative embodiment, steps (j) and (k) are not performed, so that the reduced-valence dearomatization composition becomes the final dearomatization composition, without the final acid treatment step. In another embodiment of the present invention, the dearomatization composition is made in accordance with the process described in U.S. patent application Ser. No. 10/443,380, the entire disclosure of which is incorporated herein by reference.

In accordance with one embodiment of the present invention, the dearomatization composition, described above, is employed in a process for reducing the amount of polynuclear aromatics (PNA) in a low-sulfur hydrocarbon-containing stream. Preferably, this process comprises contacting the low-sulfur hydrocarbon-containing stream with the dearomatization composition in the presence of hydrogen and under dearomatization conditions sufficient to hydrogenate at least a portion of the PNA present in the low-sulfur hydrocarbon-containing stream.

The low-sulfur hydrocarbon-containing stream employed in the dearomatization process is preferably a mixture of hydrocarbons having a boiling range (ASTM D86-00) of from about 300° F. to about 750° F., most preferably from about 350° F. to about 725° F. The low-sulfur hydrocarbon-containing stream preferably has a mid-boiling point (ASTM D86-00) of more than about 350° F., more preferably more than about 400° F., and most preferably more than about 450° F. The low-sulfur hydrocarbon-containing stream preferably has an API gravity (ASTM D287-92) in the range of from about 20 to about 50, more preferably from about 25 to about 45. The low-sulfur hydrocarbon-containing stream preferably has a minimum flash point (ASTM D93-99) of at least about 80° F., most preferably at least about 90° F.

Preferably, the low-sulfur hydrocarbon-containing stream is a middle distillate stream such as, for example, diesel fuel, jet fuel, kerosene, and/or light cycle oil. Most preferably, the low-sulfur hydrocarbon-containing stream consists essentially of diesel fuel boiling in the range of from 375° F. to 700° F., having a mid-boiling point of more than 500° F., having an API gravity in a range of from 30 to 38, and having a minimum flash point above 100° F.

The step of contacting the low-sulfur hydrocarbon-containing stream with the dearomatization composition disclosed herein, can be operated as a continuous process or, preferably, as a batch process. Preferably, the dearomatization zone is defined in a fixed bed or a moving bed reactor. Most preferably, a fixed bed reactor is employed.

The contacting of the low-sulfur hydrocarbon-containing stream and the dearomatization composition is preferably carried out under dearomatization conditions that promotes the hydrogenation of PNA compounds. Generally, such dearomatization conditions include a temperature in the range from about 350 to about 800° F., and most preferably in the range from 550 to 650° F. The dearomatization conditions also include a pressure in the range from about 200 to about 2,000 psia, and most preferably in the range from 500 to 1,000 psia. The dearomatization conditions also include the rates at which the low-sulfur hydrocarbon-containing stream and hydrogen are charged to the dearomatization zone. The flow rate of the low-sulfur hydrocarbon-containing stream can be quantified by normalizing it based on the amount of catalyst in the dearomatization zone. This normalized flow rate of the low-sulfur hydrocarbon-containing stream is expressed as a weight hourly space velocity (“WHSV”). WHSV is the numerical ratio of the rate at which the low-sulfur hydrocarbon-containing stream is charged to the dearomatization zone, in pounds per hour at standard temperature and pressure (STP), divided by the pounds of the dearomatization composition contained in the dearomatization zone. Preferably, the low-sulfur hydrocarbon-containing stream is introduced into the dearomatization zone at a WHSV in the range of from about 0.1 to about 5 hr⁻¹, and most preferably in the range of from about 0.5 to about 3 hr⁻¹. The flow rate of hydrogen to the dearomatization zone can be normalized based on the rate of the low-sulfur hydrocarbon-containing stream to the dearomatization zone by expressing the hydrogen flow rate in terms of standard cubic feet of hydrogen per barrel of low-sulfur hydrocarbon-containing stream (scf/bbl). Preferably, hydrogen is introduced into the dearomatization zone at a rate in the range of from about 1,000 to about 5,000 scf/bbl, and most preferably in the range of from 2,500 to 3,500 scf/bbl.

The contacting of the low-sulfur hydrocarbon-containing stream with the dearomatization composition in the dearomatization zone produces a PNA-reduced stream having a PNA content less than the PNA content of the low-sulfur hydrocarbon-containing stream charged to the dearomatization zone. The PNA content of the low-sulfur hydrocarbon-containing stream charged to the dearomatization zone is preferably at least about 5 weight percent, more preferably in the range of from about 10 to about 70 weight percent, and most preferably in the range of from 15 to 55 weight percent. The PNA content of the PNA-reduced stream is preferably at least about 25 percent by weight less than the PNA content of the low-sulfur hydrocarbon-containing stream, more preferably at least about 50 percent by weight less, and most preferably at least 75 percent by weight less. For example, if the PNA content of the low-sulfur hydrocarbon-containing stream is 20 weight percent and the PNA content of the PNA-reduced stream is 10 weight percent, then the PNA content of the PNA-reduced stream is 50 percent less by weight than the PNA content of the low-sulfur hydrocarbon-containing stream. Preferably, the PNA content of the PNA-reduced stream is less than about 10 weight percent, more preferably less than about 7.5 weight percent, and most preferably less than 5 weight percent.

It is further preferred for the contacting of the low-sulfur hydrocarbon-containing stream with the dearomatization composition in the dearomatization zone to reduce the total aromatics content of the stream. The total aromatics content of the low-sulfur hydrocarbon-containing stream is preferably in the range of from about 10 to about 90 weight percent, most preferably in the range of from 20 to 80 weight percent. Preferably, the total aromatics content of the PNA-reduced stream is at least about 10 percent by weight less than the total aromatics content of the low-sulfur hydrocarbon-containing stream, more preferably at least about 25 percent by weight less, and most preferably at least 50 percent by weight less.

In addition, it is preferred for the contacting of the low-sulfur hydrocarbon-containing stream with the dearomatization composition to reduce the sulfur content of the stream. The sulfur content of the low-sulfur hydrocarbon-containing stream is preferably less than about 500 ppmw, more preferably in the range of from about 5 to about 400 ppmw, and most preferably in the range of from 10 to 250 ppmw. Preferably, the sulfur content of the PNA-reduced stream is at least about 5 percent by weight less than the sulfur content of the low-sulfur hydrocarbon-containing stream, more preferably at least about 25 percent by weight less, and most preferably at least 50 percent by weight less. Preferably, the sulfur content of the PNA-reduced stream is less than about 10 ppmw, most preferably less than 5 ppmw. The term “sulfur,” as used herein, denotes sulfur in any form such as elemental sulfur or a sulfur compound normally present in a hydrocarbon-containing stream.

In addition, it is preferred for the contacting of the low-sulfur hydrocarbon-containing stream with the dearomatization composition to increase the cetane number of the stream. The cetane number of the low-sulfur hydrocarbon-containing stream is preferably at least about 20, most preferably in the range of from 20 to 40. Preferably, the cetane number of the PNA-reduced stream is at least about 5 percent greater than the cetane number of the low-sulfur hydrocarbon-containing stream, more preferably at least about 10 percent greater, and most preferably at least 25 percent greater. Preferably, the cetane number of the PNA-reduced stream is greater than about 30, most preferably in the range of from 35 to 60.

It is also preferred for the contacting of the low-sulfur hydrocarbon-containing stream with the dearomatization composition to have little effect on the total olefins content, mid-boiling point, API gravity, and flash point of the stream. Preferably, the total olefins content, mid-boiling point, API gravity, and flash point of the PNA-reduced stream is within about 10 percent of the total olefins, content mid-boiling point, API gravity, and flash point of the low-sulfur hydrocarbon-containing stream, more preferably within about 5 percent, and most preferably within 2 percent.

As mentioned above, the dearomatization composition has enhanced resistance to sulfur poisoning compared to conventional catalysts used to hydrogenate PNA compounds. Thus, the dearomatization composition maintains its dearomatization activity for long periods of time without requiring replacement and/or regeneration. This enhanced activity can be quantified with reference to an activity maintaining time period. As used herein, “activity maintaining time period” denotes a period of time during which PNA dearomatization activity is maintained above a certain level without replacing or regenerating the dearomatization composition and without changing the dearomatization conditions. Preferably, the activity-maintaining time period during which the dearomatization composition is sufficiently active to produce a PNA-reduced stream having a PNA content at least 25 percent by weight less than the PNA content of the low-sulfur hydrocarbon-containing stream is at least about 12 hours, more preferably at least about 36 hours, and most preferably at least about 72 hours. Further, the activity-maintaining time period during which the dearomatization composition is sufficiently active to produce a PNA-reduced stream having a PNA content at least 50 percent by weight less than the PNA content of the low-sulfur hydrocarbon-containing stream is preferably at least about 6 hours, more preferably at least about 12 hours, and most preferably at least 36 hours.

During dearomatization of the low-sulfur hydrocarbon-containing stream in the dearomatization zone, a portion of the zinc oxide of the dearomatization composition can be converted to zinc sulfide. This conversion of zinc oxide to zinc sulfide can contribute to the deactivation of the dearomatization compound. After the dearomatization composition has become deactivated (i.e., after the activity-maintaining time period), the deactivated dearomatization composition can be regenerated via contact with an oxygen-containing regeneration stream. Regeneration of the deactivated dearomatization compound preferably converts substantially all of the zinc sulfide back to zinc oxide. The regenerated dearomatization can then be employed once again in the dearomatization zone to reduce the PNA content of a low-sulfur hydrocarbon-containing stream.

Prior to contacting with the dearomatization composition in the dearomatization zone, it is preferred for the low-sulfur hydrocarbon-containing stream to have been subjected to desulfurization in a suitable desulfurization system. The initial hydrocarbon-contain stream introduced into the desulfurization system located upstream of the dearomatization zone preferably has a sulfur content of at least about 500 ppmw sulfur, most preferably in the range of from 600 to 5,000 ppmw. The desulfurization that takes place in the desulfurization system preferably produces a low-sulfur hydrocarbon-containing stream with a sulfur content that is at least 25 percent by weight less than the sulfur content of the initial hydrocarbon-containing stream, more preferably at least about 50 percent by weight less, and most preferably at least 75 percent by weight less. Other than the sulfur content, all other properties (e.g., boiling range, cetane number, API gravity, minimum flash point, PNA content, total aromatics content, and total olefins content) of the initial hydrocarbon-containing stream are substantially that same as the sulfur-reduced hydrocarbon-containing stream, described above.

Desulfurization of the initial hydrocarbon-containing stream can be carried out by any process know in the art for reducing the sulfur content of a hydrocarbon-containing stream. Preferred desulfurization processes include hydrodesulfurization in the presence of a hydrogenation catalyst and/or sulfur sorption in the presence of a sulfur sorbent. A suitable hydrodesulfurization process is described in U.S. Pat. No. 5,011,593, the entire disclosure of which is incorporated herein by reference. A suitable sulfur sorption process is described in U.S. Pat. App. Ser. No. 2003/0111389, the entire disclosure of which is incorporated herein by reference. Most preferably, desulfurization is carried out by sulfur sorption using a regenerable sorbent comprising a reduced-valence promoter metal component and zinc oxide. In one embodiment, the regenerable sorbent employed in the desulfurization system has substantially the same composition as the dearomatization composition, described above. The desulfurization is preferably carried out in desulfurization zone of a fluidized bed reactor.

The following examples are intended to be illustrative of the present invention and to teach one of ordinary skill in the art to make and use the invention. The examples are not intended to limit the invention in any way.

EXAMPLES Example I Composition Preparation Composition A (Control)

Composition A was obtained by incipient wetness impregnation of a silica-alumina support with an aqueous solution of nickel nitrate hexahydrate. The resulting impregnated composition was dried at 100° C. and calcined at 400° C. The concentration of the nickel metal on the final Composition A was approximately 9.4 weight percent nickel.

Composition B (Inventive)

Composition B was prepared using a base extrudate of expanded crushed perlite, zinc oxide, and alumina. This base extrudate material was impregnated with a nickel-nitrate hexahydrate solution. Immediately after the initial impregnation, the impregnated extrudate was contacted with citric acid by using a drip method of impregnation. The acid-treated composition was then dried at 100° C. and calcined at 400° C. The composition was then impregnated with a nickel nitrate solution a second time, followed by drying at 100° C. and calcining at 400° C. The concentration of the nickel metal on the final Composition B was approximately 18.1 weight percent nickel.

Composition C (Control)

Composition C was prepared by extruding a paste of perlite, zinc oxide, and alumina. The resulting extrudate was dried at 100° C. and calcined at 400° C. in a temperature-programmed muffle oven. The calcined extrudate was then impregnated by spraying an aqueous solution of nickel nitrate hexahydrate onto the extrudate using ultra-sonic nozzles. The resulting impregnated composition was dried at 100° C. and calcined at 635° C. The concentration of the nickel metal on the final Composition C was approximately 13.6 weight percent nickel.

Example II Determination of Polynuclear Aromatics Conversion

Compositions A, B, and C, as prepared in Example I, were each tested for polynuclear aromatics (PNA) conversion.

The compositions were evaluated in a laboratory-scale fixed bed reactor. The reaction conditions included a temperature of about 600° F., a pressure of about 500 psig, a weight hourly space velocity of about 2 h⁻¹, and a H₂/hydrocarbon ratio of about 3,500 scf/bbl.

The feed to the reactor was a low-sulfur diesel fuel spiked with an additional 100 ppmw of sulfur. The sulfur used to spike the diesel fuel was 50 weight percent sulfur as dibenzothiophene and 50 weight percent sulfur as 4,6-dimethyldibenzothiophene. The spiked diesel feed had a total sulfur content of 118.6 ppmw, a PNA content of 25 weight percent, and a total aromatics content of 48.4 weight percent.

Prior to contacting with the spiked diesel feed, compositions A, B, and C were reduced with hydrogen at a temperature of 750° F. for a period of three hours.

To test the activity of Compositions A, B, and C, the reactor tube was loaded with a fixed bed of the tested composition, and the spiked diesel was passed though the fixed bed under the above-described dearomatization conditions. During contacting of Compositions A, B, and C with the spiked diesel feed, the PNA percent conversion per gram of nickel was measured at regular intervals based on the amount of sulfur accumulated on the dearomatization composition.

A comparison of the PNA hydrogenation activity of Compositions A, B, and C is shown in FIG. 1. FIG. 1 shows that Composition B, the inventive composition, had superior PNA hydrogenation activity compared to Compositions A and C. Further, Composition B had superior resistance to deactivation relative to Composition A.

Example III Comparison of Composition Resistance to Deactivation

Compositions A and B, as prepared in Example I, were tested for resistance to deactivation by measuring the amount of sulfur over time in an ultra-low sulfur feed stream spiked with 100 ppm sulfur.

The compositions were evaluated in a laboratory-scale fixed bed reactor. The reaction conditions included a temperature of about 600° F., a pressure of about 500 psig, a weight hourly space velocity of about 2 h⁻¹, and a H₂/hydrocarbon ratio of about 3,500 scf/bbl.

The feed to the reactor was a low-sulfur diesel fuel spiked with an additional 100 ppmw of sulfur. The sulfur used to spike the diesel fuel was 50 weight percent sulfur as dibenzothiophene and 50 weight percent sulfur as 4,6-dimethyldibenzothiophene. The spiked diesel feed had a total sulfur content of 118.6 ppmw, a PNA content of 25 weight percent, and a total aromatics content of 48.4 weight percent.

Prior to contacting with the spiked diesel feed, compositions A and B were reduced with hydrogen at a temperature of 750° F. for a period of three hours.

To test the activity of Compositions A and B, the reactor tube was loaded with a fixed bed of the tested composition, and the spiked diesel was passed though the fixed bed under the above-described dearomatization conditions. During contacting of Compositions A and B with the spiked diesel feed, the amount of sulfur remaining in the feed stream was measured at regular intervals.

A comparison of the resistance to deactivation of Compositions A and B is shown in FIG. 2. FIG. 2 shows that Composition B, the inventive composition, had superior resistance to deactivation relative to Composition A. 

1. A process for removing polynuclear aromatics (PNA) from a low-sulfur hydrocarbon-containing stream containing less than about 500 ppmw sulfur, said process comprising: contacting said low-sulfur stream with a dearomatization composition comprising a promoter metal component and zinc oxide under dearomatization conditions sufficient to reduce the PNA content of said low-sulfur stream by at least about 25 percent by weight thereby providing a PNA-reduced stream, wherein the sulfur content of said PNA-reduced stream is at least about 5 percent by weight less than the sulfur content of said low-sulfur stream.
 2. The process of claim 1 further comprising using a desulfurization system to remove sulfur from an initial hydrocarbon-containing stream to thereby produce said low-sulfur stream, wherein the sulfur content of said low-sulfur stream is at least about 50 percent by weight less than the sulfur content of said initial hydrocarbon-containing stream.
 3. The process of claim 2 wherein said desulfurization system employs a regenerable sorbent comprising a reduced-valence promoter metal component and zinc oxide to remove sulfur from said initial hydrocarbon-containing stream.
 4. The process of claim 3 wherein said desulfurization system includes a fluidized bed reactor for contacting said initial hydrocarbon-containing stream with said regenerable sorbent.
 5. The process of claim 1 wherein said dearomatization composition is pre-treated with citric acid and/or tartaric acid prior to said contacting with said low-sulfur stream.
 6. The process of claim 1 wherein said dearomatization composition is subjected to reduction prior to said contacting with said low-sulfur stream.
 7. The process of claim 1 wherein said promoter metal component comprises a promoter metal selected from the group consisting of nickel, cobalt, iron, manganese, tungsten, silver, gold, copper, platinum, zinc, ruthenium, molybdenum, antimony, vanadium, iridium, chromium, palladium, and combinations thereof.
 8. The process of claim 7 wherein said dearomatization composition contains at least about 20 percent by weight of said zinc oxide.
 9. The process of claim 8 wherein said contacting with said low-sulfur stream converts at least a portion of said zinc oxide to zinc sulfide.
 10. The process of claim 9 wherein said dearomatization composition contains at least about 10 percent by weight nickel.
 11. The process of claim 1 wherein said promoter metal component is a substitutional solid metal solution of the formula Ni_(A)Zn_(B), wherein A and B are each in the range of from 0.01 to 0.99.
 12. The process of claim 11 wherein A is in the range of from about 0.70 to about 0.97 and B is in the range of from about 0.03 to about 0.30.
 13. The process of claim 1 wherein said dearomatization composition contains an aluminate.
 14. The process of claim 13 wherein said aluminate contains nickel and/or zinc.
 15. The process of claim 13 wherein said aluminate is nickel-zinc aluminate.
 16. The process of claim 1 wherein said dearomatization composition contains perlite.
 17. The process of claim 1 wherein the cetane number of said PNA-reduced stream is at least about 5 percent greater than the cetane number of said low-sulfur stream.
 18. The process of claim 1 wherein the PNA content of said PNA-reduced stream is at least about 50 percent by weight less than the PNA content of said low-sulfur stream, wherein the sulfur content of said PNA-reduced stream is at least about 25 percent by weight less than the sulfur content of said low-sulfur stream, wherein the cetane number of said PNA-reduced stream is at least about 10 percent greater than the cetane number of said low-sulfur stream.
 19. The process of claim 1 wherein said low-sulfur stream has a cetane number of at least about
 20. 20. The process of claim 1 wherein said low-sulfur stream has a cetane number in the range of from about 20 to about 40, wherein said PNA-reduced stream has a cetane number in the range of from about 35 to about
 60. 21. The process of claim 1 wherein said low-sulfur stream has a PNA content of at least about 5 percent by weight.
 22. The process of claim 1 wherein said low-sulfur stream has a PNA content in the range of from about 10 to about 70 percent by weight, wherein said PNA-reduced stream has a PNA content less than about 7.5 percent by weight.
 23. The process of claim 1 wherein said low-sulfur stream contains in the range of from about 5 to about 400 ppmw sulfur, wherein said PNA-reduced stream contains less than about 10 ppmw sulfur.
 24. The process of claim 1 wherein said low-sulfur stream is a middle distillate stream having a mid-boiling point of at least about 350° F.
 25. The process of claim 24 wherein said middle distillate stream has a boiling range of from about 300 to about 750° F., an API gravity in the range of from about 20 to about 50, and a minimum flashpoint of at least about 80° F.
 26. The process of claim 1 wherein said low-sulfur stream is selected from the group consisting of diesel fuel, jet fuel, kerosene, light cycle oil, and mixtures thereof.
 27. The process of claim 1 wherein said low-sulfur stream consists essentially of diesel fuel having a boiling range of 375 to 700° F., a mid-boiling point of at least 500° F., an API gravity in the range of from 30 to 38, and a minimum flashpoint of at least 100° F.
 28. The process of claim 1 wherein said contacting is carried out for an activity-maintaining time period of at least 12 hours without regeneration of said dearomatization composition, wherein during said activity-maintaining time period the reduction in the PNA content of said low-sulfur stream is maintained at 25 percent by weight or more.
 29. The process of claim 28 wherein during said activity-maintaining time period the reduction in the sulfur content of said low-sulfur stream is maintained at 50 percent by weight or more.
 30. The process of claim 28 wherein said activity-maintaining time period is at least 36 hours.
 31. The process of claim 28 wherein after said activity-maintaining time period said dearomatization composition is deactivated, wherein said process further comprises regenerating the deactivated dearomatization composition by contacting the deactivated dearomatization composition with an oxygen-containing regeneration stream.
 32. The process of claim 31 wherein said deactivated dearomatization composition contains zinc sulfide, wherein said regeneration converts at least a portion of said zinc sulfide to zinc oxide.
 33. The process of claim 1 wherein said contacting takes place in a fixed bed reactor and/or moving bed reactor.
 34. The process of claim 1 wherein said dearomatization composition is in the form of solid particles having an average minimum diameter of at least about 0.0625 inches.
 35. The process of claim 1 wherein said dearomatization conditions include a temperature in the range of from about 500 to about 800° F., a pressure in the range of from about 200 to about 2,000 psia, a weight hourly space velocity in the range of from about 0.1 to about 5 hr⁻¹, and a hydrogen flow rate in the range of from about 1,000 to about 5,000 scf/bbl.
 36. A process for treating an initial hydrocarbon-containing stream having a sulfur content of at least about 500 ppmw and a polynuclear aromatics (PNA) content of at least about 5 percent by weight, said process comprising: (a) contacting said initial stream with a catalyst and/or sorbent composition in a first zone under desulfurization conditions sufficient to reduce the sulfur content of said initial stream by at least about 25 percent by weight, thereby producing a sulfur-reduced stream having a sulfur content of less than about 500 ppmw and a PNA content of at least about 5 percent by weight; and (b) contacting at least a portion of said sulfur-reduced stream with a dearomatization composition comprising nickel and zinc oxide in a second zone, thereby producing a PNA-reduced stream having a PNA content at least about 25 percent by weight less than the PNA content of said sulfur-reduced stream.
 37. The process of claim 36 wherein step (a) is carried out in a fluidized bed reactor and step (b) is carried out in a fixed and/or moving bed reactor.
 38. The process of claim 36 wherein said catalyst and/or sorbent is a regenerable sorbent comprising a reduced-valence promoter metal component and zinc oxide.
 39. The process of claim 36 wherein said dearomatization composition comprises at least about 10 percent by weight nickel and at least about 20 percent by weight zinc oxide.
 40. The process of claim 36 wherein said dearomatization composition has been pretreated with an acid prior to said contacting of step (b).
 41. The process of claim 36 wherein said initial stream is a middle distillate stream having a mid-boiling point of at least about 350° F.
 42. The process of claim 36 wherein said initial stream is diesel fuel.
 43. The process of claim 36 wherein the sulfur content of said PNA-reduced stream is at least about 5 percent by weight less than the sulfur content of said sulfur-reduced stream.
 44. The process of claim 36 wherein the PNA content of said PNA-reduced stream is less than about 10 percent by weight.
 45. The process of claim 36 wherein the sulfur content of said sulfur-reduced stream is at least about 50 percent less than the sulfur content of said initial stream.
 46. The process of claim 45 wherein the PNA content of said PNA-reduced stream is at least about 50 percent by weight less than the PNA content of said sulfur-reduced stream.
 47. The process of claim 46 wherein the PNA content of said PNA-reduced stream is less than about 7.5 percent by weight, wherein the PNA content of said sulfur-reduced stream is in the range of from about 10 to abut 60 percent by weight. 